As a material from which a conductive layer of a substrate with a transparent conductive layer is formed, a carbon nanotube (hereinafter abbreviated as CNT) and a conductive polymer are known. When these materials are used, a conductive layer can be coated at room temperature under an atmospheric pressure and the conductive layer can be formed by a simple process. Since these materials are rich in flexibility, even when the conductive layer is formed on a flexible film, they can follow flexibility of the film. Furthermore, when a film is used as the substrate, since the conductive layer can be continuously formed, the process cost can be reduced. Transparency can be improved by decreasing the thickness of these conductive layers. Since CNT has a black color, a neutral color tone can be obtained.
Although it has hitherto been difficult to disperse CNT in a solvent, a composition containing a conductive polymer, a solvent and CNT has recently been proposed as a composition having enhanced dispersibility of CNT (for example, JP 2005-97499). It became possible to obtain a conductive layer having excellent transparency and conductivity by such a dispersion method. However, the obtained CNT layer showed insufficient tight adhesion to the substrate. Therefore, there are proposed a method in which a coating solution containing a binder resin, CNT and a solvent is coated on a surface of a substrate to form a conductive layer made of CNT (for example, JP 2004-526838) and a multi-layered structure in which a polymer layer is provided on a transparent conductive film containing CNT (for example, JP 2006-519712). The method of adding a binder resin had a problem that resistance increases since the binder resin exists at a contact point between CNT and CNT in a CNT layer extending in a mesh shape. In the method in which a coating solution containing a binder resin, CNT and a polymer layer is provided on a conductive layer made of CNT, since a CNT conductive layer has poor tight adhesion before formation of the polymer layer, the CNT conductive layer was sometimes peeled during the process.
There is also proposed a method in which a binding capacity is enhanced by providing the CNT conductive layer with a base coat, while resistance stability to humidity is enhanced by providing the CNT conductive layer with a top coat (for example, U.S. Pat. No. 7,378,040). However, there was a problem that surface resistance increased 1.25 times or more after a heat treatment at 125° C. for 2 hours, resulting in poor resistance value stability. There was also a problem that the obtained conductive layer has insufficient in-plane uniformity.
It could therefore be helpful to provide a substrate with a transparent conductive layer having a conductive layer, which is excellent in tight adhesion to a substrate and in-plane uniformity.